1. Field of the Invention
The present invention relates to a solid-state image sensor for outputting electric signals corresponding to amounts of light incident thereto and an image pickup apparatus using the same. More particularly, the present invention relates to a solid-state image sensor with a color filter fitted thereon.
2. Description of the Background Art
Today, a digital camera of the type using a solid-state image sensor configured to transform an optical image captured from a scene to electric image signals representative of the image is extensively used as an image pickup apparatus. This type of camera includes a digital signal processing circuit for processing digital image signals converted from the electric image signals. The digital image signals are subjected to various kinds of correction and then stored into a memory card or similar storage mounted on the camera.
The image sensor is implemented as, e.g. a single CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) image sensor. An R (red), G (green) and B (blue) color filter is fitted on the photosensitive cell array of the image sensor and has R, G and B filter segments arranged in a Bayer or stripe pattern by way of example. With such a primary-color filter, the image sensor dot-sequentially outputs R, G and B image signals on a pixel-by-pixel basis depending upon the color pattern of the filter.
Generally, the digital camera is operated to pick up scenes illuminated under various kinds of light sources. The light sources include sunlight and artificial light sources including a flashbulb, a tungsten lamp, a fluorescent lamp and a mercury lamp. Further, the intensity of sunlight changes from the morning to the evening, from fine weather to cloudy or rainy weather, and from sunshine to shade. Before a shot, exposure adequate for a scene illuminated with any one of such light sources is determined in terms of an iris opening and a shutter speed. However, the problem with the camera is that the color balance of an image picked up is dependent on the color temperature of the light source used and other pickup conditions. With photosensitive film cameras, a traditional measure for coping with the difference in the kind of a light source is to use a color film matching with the color temperature of a light source or a color filter for adjusting color balance. By contrast, the digital camera of the type outputting electric image signals can process the image signals to adjust the white balance of image signals.
It has been customary with the digital camera to compensate for differences in sensitivity between the R, G and B color components ascribable to the color temperature of a light source by utilizing the nature of the dynamic range of the image sensor. More specifically, the R, G and B filter segments of the color filter are different in spectral transmission characteristic and therefore in transmissivity for incident light of a particular color component, resulting in differences in sensitivity between the pixels of different colors of the image sensor.
In practice, the digital camera is constructed to measure the lightness of a desired scene, evaluate an exposure time that does not bring about overexposure, and then execute automatic (or manual) exposure control for controlling the amount of exposure in terms of an iris opening and a shutter speed. Subsequently, the levels of the R, G and B component signals are adjusted to effect white balance adjustment. At this instant, to achieve sufficient tonality with a color component lower in sensitivity than the other color components, high-frequency signal components are compressed and the dynamic range is provided with a margin to prevent the signal charges from saturating. The margin of the dynamic range thus provided, however, prevents the overall dynamic range from being efficiently available with the image sensor.
Moreover, the margin of the dynamic range makes it difficult to simply reduce the size of the individual photosensitive cells of the image sensor or to drive the cells with lower voltage. Consequently, noise generated in the cells cannot be reduced.